The human heart relies on precise electrical signals to coordinate its pumping action, ensuring blood circulates effectively. When these electrical impulses become chaotic or cease, the heart’s ability to pump blood is compromised, leading to dangerous abnormal rhythms known as arrhythmias. In certain life-threatening situations where the heart’s electrical activity is severely disrupted, an emergency electrical shock can be a necessary intervention to reset the heart.
How the Heart’s Electrical System Works
The heart’s electrical system begins with a natural pacemaker, the sinoatrial (SA) node, located in the upper right chamber. This specialized tissue generates regular electrical impulses, typically between 60 and 100 times per minute in a resting adult. These impulses then spread across the upper chambers, the atria, causing them to contract and push blood into the lower chambers.
The electrical signal then travels to the atrioventricular (AV) node, which briefly delays the impulse before sending it to the ventricles. This delay allows the ventricles to fill with blood before they contract. The signal then spreads through the ventricles, causing them to contract and pump blood to the lungs and the rest of the body. Disruptions to this organized electrical pathway can lead to various abnormal heart rhythms.
Life-Threatening Rhythms Requiring a Shock
Two primary life-threatening heart rhythms necessitate an emergency electrical shock, known as defibrillation. One is ventricular fibrillation (VF), where the heart’s ventricles quiver chaotically instead of contracting in a coordinated manner. This disorganized electrical activity prevents the heart from effectively pumping blood, resulting in immediate loss of consciousness and a lack of pulse.
The other rhythm requiring a shock is pulseless ventricular tachycardia (pVT), a very fast, yet somewhat organized, electrical rhythm originating in the ventricles. Despite electrical activity, the heart beats so rapidly and inefficiently that it cannot fill with blood or produce a detectable pulse. Both VF and pVT can rapidly lead to sudden cardiac arrest if not treated promptly. The electrical shock works by momentarily stunning the entire heart, depolarizing the muscle cells simultaneously, allowing the heart’s natural pacemaker to restart with a normal, organized rhythm.
Rhythms Not Treated by Electrical Shock
Not all cardiac arrest rhythms respond to an electrical shock, as the problem may not be an electrical disorganization that can be reset. Asystole, often referred to as “flatline,” represents a complete absence of electrical activity in the heart. Since there is no electrical activity, a shock would have nothing to reset and is ineffective. Attempting to shock asystole is not a standard medical practice and is often depicted inaccurately in popular media.
Another non-shockable rhythm is pulseless electrical activity (PEA), where the heart monitor shows organized electrical activity, but the heart muscles are not contracting effectively enough to generate a pulse or pump blood. In PEA, the issue is not a chaotic electrical signal that needs to be reset, but rather a mechanical or chemical problem preventing the heart from pumping despite electrical impulses. For asystole and PEA, treatment focuses on high-quality cardiopulmonary resuscitation (CPR) and identifying and addressing underlying causes, such as severe blood loss or electrolyte imbalances, rather than delivering an electrical shock.
The Critical Role of Automated External Defibrillators
Automated External Defibrillators (AEDs) are portable, user-friendly devices designed to deliver electrical shocks for shockable heart rhythms. An AED analyzes the heart’s rhythm through electrode pads placed on the chest and advises whether a shock is needed. If a shockable rhythm like ventricular fibrillation or pulseless ventricular tachycardia is detected, the device guides the user to deliver a controlled electrical shock.
AEDs are designed for use by laypersons, with clear audio and visual commands. The immediate use of an AED improves survival rates in sudden cardiac arrest due to shockable rhythms. For every minute defibrillation is delayed, the chance of survival can decrease by approximately 7% to 10%. The increasing availability of AEDs in public places enhances the ability of bystanders to provide immediate intervention, which increases a person’s chance of survival.